Heretofore, zoom lenses having four lens groups are widely known as general zoom lenses installed on image pickup devices, such as consumer video cameras, surveillance cameras, and the like. The zoom lens includes a first lens group having a positive power, a second lens group having a negative power, a third lens group having a positive power, and a fourth lens group having a positive power arranged in this order from the object side and, when zooming, while the second lens group is moved in a optical axis direction, the fourth lens group is moved in order to correct the position of image plane associated with the zooming and to bring the zoom lens into focus. Further, a zoom lens of five lens groups in which a fifth lens group is added as a fixed lens group is also known.
A strong demand exists in the image pickup device described above to increase the zoom ratio while maintaining or even reducing the current size. For example, in monitoring applications and the like, there may be a case in which the image pickup device is required to conform to a specific standard size with enhanced specifications. There may also be a case in which there is no choice but to sacrifice somewhat optical performance when a specification, such as the zoom ratio, is enhanced. In such a case, it is important to assess and determine optical properties which should be prioritized and those allowed to be degraded to a certain extent, and to combine each component such that appropriate performance may be obtained.
For example, in zoom lenses of four-group or five-group configuration, if an attention is paid to lens configurations from the third and a subsequent lens groups, third lens groups configured with a less number of lenses, i.e., with two lenses are known as described, for example, in Japanese Patent Nos. 4194876 and 4447703. The third lens group described above is configured with two lenses: a positive lens and a negative lens having a meniscus shape with a concave surface on the image side. Further, the materials used for the two lenses are quite similar, in which a material having a refractive index of about 1.6 with an Abbe number of about 59 is used for the positive lens and a material having a refractive index of about 1.85 with an Abbe number of about 24 is used for the negative lens. Third lens groups having a similar configuration to that described above are commonly found and a relatively low dispersion material is generally used for the positive lens, as in the aforementioned example.
Low dispersion materials have also a low refractive index and, inmost cases, materials having a refractive index of not greater than 1.6 are used, as described above. But, there may be a case in which the positive lens of the third lens group is formed using a relatively high refractive index material. For example, a zoom lens in which the positive lens of the third lens group is formed using a material having a refractive index of 1.74 with an Abbe number of 44.9 is known as described, for example, in Japanese Unexamined Patent Publication No. 2000-321495. Further, as a very rare case, a zoom lens in which the positive lens of the third lens group is formed using a material having a refractive index greater than 1.8 is also known as described, for example, in Japanese Patent No. 3800847 and Japanese Unexamined Patent Publication No, 2000-081572.